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Water is arguably the most important substance on and in the Earth. Early civilizations developed and flourished only where sufficient supplies of fresh water for human consumption and to support agriculture existed. Moreover, water plays an important role in the geological, geochemical, and geodynamical evolution of our planet. The whole Earth geohydrologic cycle describes the occurrence and movement of water in the Earth system, from the clouds to the core. Reservoirs that comprise the conventional hydrologic cycle define the exosphere, whereas those reservoirs that are part of the solid Earth represent the geosphere. Exosphere reservoirs thus include the atmosphere, the oceans, surface water, groundwater, the biosphere and glaciers and polar ice. Continental crust, oceanic crust, upper mantle, transition zone, lower mantle and the core make up the geosphere. The exosphere and geosphere are linked through the active plate tectonic processes of subduction and volcanism.
The storage capacities of reservoirs in the geosphere have been reasonably well constrained by experimental and observation studies, but much uncertainty exists concerning the amount of water actually held in the geosphere. Assuming that the amount of water in the upper mantle, transition zone and lower mantle represents 10%, 10% and 50% of their storage capacities, respectively, the total amount of water in the Earth’s mantle (1.2 × 1021 kg) is comparable to the amount of water held in the world’s oceans (1.37 × 1021 kg).
Fluxes between reservoirs in the geohydrologic cycle vary by ~7 orders of magnitude, and range from 4.25 × 1017 kg/yr between the oceans and atmosphere to 5 × 1010 kg/yr between the lower mantle and transition zone. Residence times for water in the various reservoirs of the geohydrologic cycle also show wide variation, and range from 2.6 × 10-2 yr (about 10 days) for water in the atmosphere, to 6.6 × 109 yr for water in the transition zone.